defmodule Plug.Router do
@moduledoc ~S"""
A DSL to define a routing algorithm that works with Plug.
It provides a set of macros to generate routes. For example:
defmodule AppRouter do
use Plug.Router
plug :match
plug :dispatch
get "/hello" do
send_resp(conn, 200, "world")
end
match _ do
send_resp(conn, 404, "oops")
end
end
Each route receives a `conn` variable containing a `Plug.Conn`
struct and it needs to return a connection, as per the Plug spec.
A catch-all `match` is recommended to be defined as in the example
above, otherwise routing fails with a function clause error.
The router is itself a plug, which means it can be invoked as:
AppRouter.call(conn, AppRouter.init([]))
Each `Plug.Router` has a plug pipeline, defined by `Plug.Builder`,
and by default it requires two plugs: `:match` and `:dispatch`.
`:match` is responsible for finding a matching route which is
then forwarded to `:dispatch`. This means users can easily hook
into the router mechanism and add behaviour before match, before
dispatch, or after both. See the `Plug.Builder` module for more
information.
## Routes
get "/hello" do
send_resp(conn, 200, "world")
end
In the example above, a request will only match if it is a `GET`
request and the route is "/hello". The supported HTTP methods are
`get`, `post`, `put`, `patch`, `delete` and `options`.
A route can also specify parameters which will then be available
in the function body:
get "/hello/:name" do
send_resp(conn, 200, "hello #{name}")
end
This means the name can also be used in guards:
get "/hello/:name" when name in ~w(foo bar) do
send_resp(conn, 200, "hello #{name}")
end
The `:name` parameter will also be available in the function body as
`conn.params["name"]` and `conn.path_params["name"]`.
The identifier always starts with `:` and must be followed by letters,
numbers, and underscores, like any Elixir variable. It is possible for
identifiers to be either prefixed or suffixed by other words. For example,
you can include a suffix such as a dot delimited file extension:
get "/hello/:name.json" do
send_resp(conn, 200, "hello #{name}")
end
The above will match `/hello/foo.json` but not `/hello/foo`.
Other delimiters such as `-`, `@` may be used to denote suffixes.
Routes allow for globbing which will match the remaining parts
of a route. A glob match is done with the `*` character followed
by the variable name. Typically you prefix the variable name with
underscore to discard it:
get "/hello/*_rest" do
send_resp(conn, 200, "matches all routes starting with /hello")
end
But you can also assign the glob to any variable. The contents will
always be a list:
get "/hello/*glob" do
send_resp(conn, 200, "route after /hello: #{inspect glob}")
end
Opposite to `:identifiers`, globs do not allow prefix nor suffix
matches.
Finally, a general `match` function is also supported:
match "/hello" do
send_resp(conn, 200, "world")
end
A `match` will match any route regardless of the HTTP method.
Check `match/3` for more information on how route compilation
works and a list of supported options.
## Parameter Parsing
Handling request data can be done through the
[`Plug.Parsers`](https://hexdocs.pm/plug/Plug.Parsers.html#content) plug. It
provides support for parsing URL-encoded, form-data, and JSON data as well as
providing a behaviour that others parsers can adopt.
Here is an example of `Plug.Parsers` can be used in a `Plug.Router` router to
parse the JSON-encoded body of a POST request:
defmodule AppRouter do
use Plug.Router
plug :match
plug Plug.Parsers,
parsers: [:json],
pass: ["application/json"],
json_decoder: Jason
plug :dispatch
post "/hello" do
IO.inspect conn.body_params # Prints JSON POST body
send_resp(conn, 200, "Success!")
end
end
It is important that `Plug.Parsers` is placed before the `:dispatch` plug in
the pipeline, otherwise the matched clause route will not receive the parsed
body in its `Plug.Conn` argument when dispatched.
`Plug.Parsers` can also be plugged between `:match` and `:dispatch` (like in
the example above): this means that `Plug.Parsers` will run only if there is a
matching route. This can be useful to perform actions such as authentication
*before* parsing the body, which should only be parsed if a route matches
afterwards.
## Error handling
In case something goes wrong in a request, the router by default
will crash, without returning any response to the client. This
behaviour can be configured in two ways, by using two different
modules:
* `Plug.ErrorHandler` - allows the developer to customize exactly
which page is sent to the client via the `handle_errors/2` function;
* `Plug.Debugger` - automatically shows debugging and request information
about the failure. This module is recommended to be used only in a
development environment.
Here is an example of how both modules could be used in an application:
defmodule AppRouter do
use Plug.Router
if Mix.env == :dev do
use Plug.Debugger
end
use Plug.ErrorHandler
plug :match
plug :dispatch
get "/hello" do
send_resp(conn, 200, "world")
end
defp handle_errors(conn, %{kind: _kind, reason: _reason, stack: _stack}) do
send_resp(conn, conn.status, "Something went wrong")
end
end
## Passing data between routes and plugs
It is also possible to assign data to the `Plug.Conn` that will
be available to any plug invoked after the `:match` plug.
This is very useful if you want a matched route to customize how
later plugs will behave.
You can use `:assigns` (which contains user data) or `:private`
(which contains library/framework data) for this. For example:
get "/hello", assigns: %{an_option: :a_value} do
send_resp(conn, 200, "world")
end
In the example above, `conn.assigns[:an_option]` will be available
to all plugs invoked after `:match`. Such plugs can read from
`conn.assigns` (or `conn.private`) to configure their behaviour
based on the matched route.
## `use` options
All of the options given to `use Plug.Router` are forwarded to
`Plug.Builder`. See the `Plug.Builder` module for more information.
## Telemetry
The router emits the following telemetry events:
* `[:plug, :router_dispatch, :start]` - dispatched before dispatching to a matched route
* Measurement: `%{system_time: System.system_time}`
* Metadata: `%{telemetry_span_context: term(), conn: Plug.Conn.t, route: binary, router: module}`
* `[:plug, :router_dispatch, :exception]` - dispatched after exceptions on dispatching a route
* Measurement: `%{duration: native_time}`
* Metadata: `%{telemetry_span_context: term(), conn: Plug.Conn.t, route: binary, router: module, kind: :throw | :error | :exit, reason: term(), stacktrace: list()}`
* `[:plug, :router_dispatch, :stop]` - dispatched after successfully dispatching a matched route
* Measurement: `%{duration: native_time}`
* Metadata: `%{telemetry_span_context: term(), conn: Plug.Conn.t, route: binary, router: module}`
"""
@doc false
defmacro __using__(opts) do
quote location: :keep do
import Plug.Router
@plug_router_to %{}
@before_compile Plug.Router
use Plug.Builder, unquote(opts)
@doc false
def match(conn, _opts) do
do_match(conn, conn.method, Plug.Router.Utils.decode_path_info!(conn), conn.host)
end
@doc false
def dispatch(%Plug.Conn{} = conn, opts) do
{path, fun} = Map.fetch!(conn.private, :plug_route)
try do
:telemetry.span(
[:plug, :router_dispatch],
%{conn: conn, route: path, router: __MODULE__},
fn ->
conn = fun.(conn, opts)
{conn, %{conn: conn, route: path, router: __MODULE__}}
end
)
catch
kind, reason ->
Plug.Conn.WrapperError.reraise(conn, kind, reason, __STACKTRACE__)
end
end
defoverridable match: 2, dispatch: 2
end
end
@doc false
defmacro __before_compile__(env) do
unless Module.defines?(env.module, {:do_match, 4}) do
raise "no routes defined in module #{inspect(env.module)} using Plug.Router"
end
router_to = Module.get_attribute(env.module, :plug_router_to)
init_mode = Module.get_attribute(env.module, :plug_builder_opts)[:init_mode]
defs =
for {callback, {mod, opts}} <- router_to do
if init_mode == :runtime do
quote do
defp unquote(callback)(conn, _opts) do
unquote(mod).call(conn, unquote(mod).init(unquote(Macro.escape(opts))))
end
end
else
opts = mod.init(opts)
quote do
defp unquote(callback)(conn, _opts) do
require unquote(mod)
unquote(mod).call(conn, unquote(Macro.escape(opts)))
end
end
end
end
quote do
unquote_splicing(defs)
import Plug.Router, only: []
end
end
@doc """
Returns the path of the route that the request was matched to.
"""
@spec match_path(Plug.Conn.t()) :: String.t()
def match_path(%Plug.Conn{} = conn) do
{path, _fun} = Map.fetch!(conn.private, :plug_route)
path
end
## Match
@doc """
Main API to define routes.
It accepts an expression representing the path and many options
allowing the match to be configured.
The route can dispatch either to a function body or a Plug module.
## Examples
match "/foo/bar", via: :get do
send_resp(conn, 200, "hello world")
end
match "/baz", to: MyPlug, init_opts: [an_option: :a_value]
## Options
`match/3` and the other route macros accept the following options:
* `:host` - the host which the route should match. Defaults to `nil`,
meaning no host match, but can be a string like "example.com" or a
string ending with ".", like "subdomain." for a subdomain match.
* `:private` - assigns values to `conn.private` for use in the match
* `:assigns` - assigns values to `conn.assigns` for use in the match
* `:via` - matches the route against some specific HTTP method(s) specified
as an atom, like `:get` or `:put`, or a list, like `[:get, :post]`.
* `:do` - contains the implementation to be invoked in case
the route matches.
* `:to` - a Plug that will be called in case the route matches.
* `:init_opts` - the options for the target Plug given by `:to`.
A route should specify only one of `:do` or `:to` options.
"""
defmacro match(path, options, contents \\ []) do
compile(nil, path, options, contents, __CALLER__)
end
@doc """
Dispatches to the path only if the request is a GET request.
See `match/3` for more examples.
"""
defmacro get(path, options, contents \\ []) do
compile(:get, path, options, contents, __CALLER__)
end
@doc """
Dispatches to the path only if the request is a HEAD request.
See `match/3` for more examples.
"""
defmacro head(path, options, contents \\ []) do
compile(:head, path, options, contents, __CALLER__)
end
@doc """
Dispatches to the path only if the request is a POST request.
See `match/3` for more examples.
"""
defmacro post(path, options, contents \\ []) do
compile(:post, path, options, contents, __CALLER__)
end
@doc """
Dispatches to the path only if the request is a PUT request.
See `match/3` for more examples.
"""
defmacro put(path, options, contents \\ []) do
compile(:put, path, options, contents, __CALLER__)
end
@doc """
Dispatches to the path only if the request is a PATCH request.
See `match/3` for more examples.
"""
defmacro patch(path, options, contents \\ []) do
compile(:patch, path, options, contents, __CALLER__)
end
@doc """
Dispatches to the path only if the request is a DELETE request.
See `match/3` for more examples.
"""
defmacro delete(path, options, contents \\ []) do
compile(:delete, path, options, contents, __CALLER__)
end
@doc """
Dispatches to the path only if the request is an OPTIONS request.
See `match/3` for more examples.
"""
defmacro options(path, options, contents \\ []) do
compile(:options, path, options, contents, __CALLER__)
end
@doc """
Forwards requests to another Plug. The `path_info` of the forwarded
connection will exclude the portion of the path specified in the
call to `forward`. If the path contains any parameters, those will
be available in the target Plug in `conn.params` and `conn.path_params`.
## Options
`forward` accepts the following options:
* `:to` - a Plug the requests will be forwarded to.
* `:init_opts` - the options for the target Plug. It is the preferred
mechanism for passing options to the target Plug.
* `:host` - a string representing the host or subdomain, exactly like in
`match/3`.
* `:private` - values for `conn.private`, exactly like in `match/3`.
* `:assigns` - values for `conn.assigns`, exactly like in `match/3`.
If `:init_opts` is undefined, then all remaining options are passed
to the target plug.
## Examples
forward "/users", to: UserRouter
Assuming the above code, a request to `/users/sign_in` will be forwarded to
the `UserRouter` plug, which will receive what it will see as a request to
`/sign_in`.
forward "/foo/:bar/qux", to: FooPlug
Here, a request to `/foo/BAZ/qux` will be forwarded to the `FooPlug`
plug, which will receive what it will see as a request to `/`,
and `conn.params["bar"]` will be set to `"BAZ"`.
Some other examples:
forward "/foo/bar", to: :foo_bar_plug, host: "foobar."
forward "/baz", to: BazPlug, init_opts: [plug_specific_option: true]
"""
defmacro forward(path, options) do
quote bind_quoted: [path: path, options: options] do
{target, options} = Keyword.pop(options, :to)
{options, plug_options} = Keyword.split(options, [:via, :host, :private, :assigns])
plug_options = Keyword.get(plug_options, :init_opts, plug_options)
if is_nil(target) or not is_atom(target) do
raise ArgumentError, message: "expected :to to be an alias or an atom"
end
{target, target_opts} =
case Atom.to_string(target) do
"Elixir." <> _ -> {target, target.init(plug_options)}
_ -> {{__MODULE__, target}, plug_options}
end
@plug_forward_target target
@plug_forward_opts target_opts
# Delegate the matching to the match/3 macro along with the options
# specified by Keyword.split/2.
match path <> "/*glob", options do
Plug.forward(
var!(conn),
var!(glob),
@plug_forward_target,
@plug_forward_opts
)
end
end
end
## Match Helpers
@doc false
def __route__(method, path, guards, options) do
{method, guards} = build_methods(List.wrap(method || options[:via]), guards)
{params, match, guards, post_match} = Plug.Router.Utils.build_path_clause(path, guards)
params = Plug.Router.Utils.build_path_params_match(params)
private = extract_merger(options, :private)
assigns = extract_merger(options, :assigns)
host_match = Plug.Router.Utils.build_host_match(options[:host])
{quote(do: conn), method, match, post_match, params, host_match, guards, private, assigns}
end
@doc false
def __put_route__(conn, path, fun) do
Plug.Conn.put_private(conn, :plug_route, {append_match_path(conn, path), fun})
end
defp append_match_path(%Plug.Conn{private: %{plug_route: {base_path, _}}}, path) do
base_path <> path
end
defp append_match_path(%Plug.Conn{}, path) do
path
end
# Entry point for both forward and match that is actually
# responsible to compile the route.
defp compile(method, expr, options, contents, caller) do
{callback, options} =
cond do
Keyword.has_key?(contents, :do) ->
{wrap_function_do(contents[:do]), expand_options(options, caller)}
Keyword.has_key?(options, :do) ->
{body, options} = Keyword.pop(options, :do)
{wrap_function_do(body), expand_options(options, caller)}
options[:to] ->
options = expand_options(options, caller)
callback =
quote unquote: false do
&(unquote(callback) / 2)
end
options =
quote do
{callback, options} = Plug.Router.__to__(unquote(caller.module), unquote(options))
options
end
{callback, options}
true ->
raise ArgumentError, message: "expected one of :to or :do to be given as option"
end
{path, guards} = extract_path_and_guards(expr)
quote bind_quoted: [
method: method,
path: path,
options: options,
guards: Macro.escape(guards, unquote: true),
callback: Macro.escape(callback, unquote: true)
] do
route = Plug.Router.__route__(method, path, guards, options)
{conn, method, match, post_match, params, host, guards, private, assigns} = route
defp do_match(unquote(conn), unquote(method), unquote(match), unquote(host))
when unquote(guards) do
unquote_splicing(post_match)
unquote(private)
unquote(assigns)
params = unquote({:%{}, [], params})
merge_params = fn
%Plug.Conn.Unfetched{} -> params
fetched -> Map.merge(fetched, params)
end
conn = update_in(unquote(conn).params, merge_params)
conn = update_in(conn.path_params, merge_params)
Plug.Router.__put_route__(conn, unquote(path), unquote(callback))
end
end
end
@doc false
def __to__(module, options) do
{to, options} = Keyword.pop(options, :to)
{init_opts, options} = Keyword.pop(options, :init_opts, [])
router_to = Module.get_attribute(module, :plug_router_to)
callback = :"plug_router_to_#{map_size(router_to)}"
router_to = Map.put(router_to, callback, {to, init_opts})
Module.put_attribute(module, :plug_router_to, router_to)
{Macro.var(callback, nil), options}
end
defp wrap_function_do(body) do
quote do
fn var!(conn), var!(opts) ->
_ = var!(opts)
unquote(body)
end
end
end
defp expand_options(opts, caller) do
if Macro.quoted_literal?(opts) do
Macro.prewalk(opts, &expand_alias(&1, caller))
else
opts
end
end
defp expand_alias({:__aliases__, _, _} = alias, env),
do: Macro.expand(alias, %{env | function: {:init, 1}})
defp expand_alias(other, _env), do: other
defp extract_merger(options, key) when is_list(options) do
if option = Keyword.get(options, key) do
quote do
conn = update_in(conn.unquote(key), &Map.merge(&1, unquote(Macro.escape(option))))
end
end
end
# Convert the verbs given with `:via` into a variable and guard set that can
# be added to the dispatch clause.
defp build_methods([], guards) do
{quote(do: _), guards}
end
defp build_methods([method], guards) do
{Plug.Router.Utils.normalize_method(method), guards}
end
defp build_methods(methods, guards) do
methods = Enum.map(methods, &Plug.Router.Utils.normalize_method(&1))
var = quote do: method
guards = join_guards(quote(do: unquote(var) in unquote(methods)), guards)
{var, guards}
end
defp join_guards(fst, true), do: fst
defp join_guards(fst, snd), do: quote(do: unquote(fst) and unquote(snd))
# Extract the path and guards from the path.
defp extract_path_and_guards({:when, _, [path, guards]}), do: {extract_path(path), guards}
defp extract_path_and_guards(path), do: {extract_path(path), true}
defp extract_path({:_, _, var}) when is_atom(var), do: "/*_path"
defp extract_path(path), do: path
end
